i am working on a 94 grand cherokee limited, one day it started to slip, so i changed the filter and tranny fluid. then i had my gears back! but when the tranny warms up the over drive shuts off and will not turn back on. and it will start to shift hard. ther was no shavings in the pan. when i ran the check engine it gave me code 12 . can some one help me please ?

   Don